Session FF3: Modeling - Thrusters and Wave-Plasma Interactions

Dielectric boundary for an unstructured 2D radial-axial fluid simulation of a Hall thruster

Following the gain of interest in Hall thrusters in recent years, a need for accurate and fast numerical simulations has arisen. Multi-fluid models solving the Euler equations for each plasma species with self-consistent electric field and chemical source terms were found to be promising to recover global plasma parameters if closure terms account for kinetic effects. However, previous Hall thruster simulations have shown that the macroscopic plasma parameters are very sensitive to the boundary conditions, especially in the radial direction, where the commonly used metallic wall condition fails to represent the behavior of the dielectric ceramic used in thrusters. To improve the simulation, a dielectric boundary condition is therefore necessary. This work presents the development and validation of a dielectric wall boundary condition for the unstructured multi-fluid solver AVIP. The new condition is first validated on analytical cases, then an extensive evaluation is performed for a 2D radial-axial configuration with comparisons to the metallic wall boundary condition and PIC simulations with dielectrics. Results have shown a significant improvement on the channel modeling and macroscopic plasma parameters.   

Chemistry of low-pressure iodine plasmas

Since a few years, Xenon which is commonly used as propellant for electric propulsion applications becomes resource-critical. Among the potential alternatives to xenon, iodine could play a key role in the coming years. However, the chemistry of iodine plasmas is much more complex than the one of Xenon with excited molecules and atoms, atomic and molecular positive and negative ions and electrons. At LPP, since a few years, low-pressure iodine plasmas in conditions relevant for electric propulsion applications have been studied experimentally by combining different electrical and optical diagnostics. To compare with these experimental data, a global model and a particle-in-cell code have been developed to simulate low-pressure iodine plasmas. For both modeling approaches, the key part is the set of reactions and cross sections. In this work, we present and discuss the current reaction set for low-pressure iodine plasmas. First, we carry out a comparison with the reaction sets of two other gases which have been studied for many years: chlorine and oxygen. Second, we use the comparison with experiments in low-pressure iodine plasmas to identify key processes in the current reaction set of iodine that need to be further investigated.

    

Deep Learning based approach for investigating Electromagnetic Wave Propagation in Plasmas

2D simulations of EM wave-plasma interaction leading to complex scattering patterns are computationally expensive and challenging. The scattering pattern is primarily determined by the plasma density profile and the incident wave frequency. We propose a novel U-Net based deep learning (DL) approach, with a series of encoder and decoder units

with skip connections, for studying microwave-plasma interaction. The scattering of a plane EM wave with fixed frequency (1 GHz ) and amplitude incident on a plasma medium with different Gaussian density profiles (leading to absorption, reflection and transmission) have been considered in this work. Training data has been generated using conventional FDTD based method. We evaluate the results obtained from the network, using various metrics like SSIM index, average percent error and mean square error, with the data obtained from well-established FDTD based technique. The trained DL model is able to reproduce the scattering pattern of the EM wave from different plasma profiles with an error margin of less than 2%. Ablation studies along with network visualization using feature maps has also been performed. The DL technique proposed in this work is significantly fast as compared to the existing computational techniques and can be used as an alternative computational approach for investigating EM-plasma interaction.   

PIC simulation of plasma sources for the on-ground reproduction of orbital flows

Ground testing of satellite aerodynamics in Very Low Earth Orbit (VLEO) requires facilities able to produce a rarefied flow of particles at hypersonic speeds up to 8 km/s. A plasma source can produce the required flow by electrostatic acceleration of ions. Electric thrusters for space propulsion can be repurposed for this application. We use Particle-in-Cell (PIC) to simulate these devices and the plasma plume they produce. Traditional explicit PIC schemes need to resolve the smallest electron length- and time scales, or numerical instabilities will appear. This requirement can make simulation of the device extremely expensive, unless an appropriate scaling can be applied. Recently, a new class of semi-implicit methods have been devised, mostly for applications in astrophysics. These feature exact total energy conservation, and are stable for cell sizes and time steps well in excess of the Debye length and plasma period, allowing the simulation of dense plasma in large domains. We investigate the applicability of such schemes to the simulation of the plasma plume of an Hall effect thruster. We discuss the advantages and disadvantages of adopting a semi-implicit scheme, and evaluate the accuracy of the results, also by comparing them with quasineutral plasma fluid solutions.   

Development and validation of iodine plasma models for electric propulsion systems

Iodine has emerged as an attractive alternative propellant to xenon for electric propulsion systems due to its significantly lower cost and its ability to be stored unpressurized as a solid. However, iodine plasmas exhibit more complex reaction processes and have additional energy-loss pathways, such as molecular dissociation, that complicate the development of theoretical or numerical plasma models. This is further exacerbated by the lack of reliable iodine electron-neutral collision cross-sections and experimental thruster performance data within the literature. Here we present a new iodine plasma model based on recently available collision cross-sections, and we perform a detailed comparison with experimental data from an iodine-fuelled gridded ion thruster. The model and experiment are in good agreement over a wide range of operating conditions and additionally show that the use of iodine can lead to a performance enhancement when compared with xenon. The results confirm the validity of the iodine reaction set used within the model which is not only relevant to electric propulsion systems, but also low-pressure iodine plasmas used in other applications.   

PIC modeling of iodine plasma for electric propulsion conditions

With xenon, the foremost propellant for electric spacecraft propulsion,  getting scarcer and more costly, some satellite manufacturers have moved to more easily sourced noble gases, such as krypton.  However, these alternative propellants have the drawback of a poorer energy efficiency, as their ionization energies are higher and ion masses lower. Iodine is a promising alternative thanks to its close physical properties to xenon.

However Iodine plasma chemistry may impact performances in ways that may be difficult to predict. Electron collisions with I₂ molecules introduce new energy losses and new charged species in the form of molecular and negative ions. Moreover, the cross sections of the reactions that occur in iodine plasmas are not as well benchmarked as in noble gases.

To simulate iodine plasma discharges we use a global model to first  compute the expected average densities and temperatures of all species species. These parameters are given as inputs to a Particle In Cell Code with a Monte-Carlo Collision module including a detailed iodine chemistry . This method allows us to test our models and cross section sets against Langmuir probe and optical measurements conducted in low pressure iodine discharges performed in a prototype thruster at LPP.   

2D axisymmetric Particle-In-Cell study of a hollow cathode and its near plume region

Hollow cathodes are efficient plasma sources and have been used in a wide variety of applications including electric propulsion, surface processing and plasma-material interaction studies.

Despite continuous progress, nonclassical processes such as the anomalous resistivity remain difficult to predict in hollow cathodes. Indeed, current modelings that are often based on a fluid or hybrid approach, rely on semi-empirical data and so need to be adjusted for each new configuration. Besides, the large mean free path downstream the orifice continues to challenge any fluid model.

In contrast, a Particle-In-Cell (PIC) method is well suited in this low-pressure regime and so it can help improve the current understanding of the plume dynamics that is highly dependent on the cathode orifice.

Therefore, in this work, we propose a fully kinetic axisymmetric two-dimensional study of a hollow cathode and its near plume region. The state-of-the-art open source code EDIPIC is used (https://github.com/PrincetonUniversity/EDIPIC-2D). Numerical results are consistent plasma expansion beams solutions, and we explore the influence of an imposed magnetic field on the plasma topology. Finally, we also investigate the impact of the orifice length-to-diameter aspect ratio on the plume dynamics.